An image sensor is a semiconductor device to convert an optical image into an electric signal. Image sensors are mainly classified as charge coupled device (CCD) image sensors and complementary metal oxide silicon (CMOS) image sensors (CIS).
The CMOS image sensor employs a switching mode to sequentially detect an output by providing MOS transistors corresponding to the number of pixels. CMOS technology is capable of integrating peripheral devices, such as a control circuit and a signal processing circuit. The CMOS image sensor cell includes a photodiode and a plurality of MOS transistors.
FIG. 1 is a sectional view showing a related CMOS image sensor. Referring to FIG. 1, an ion implantation process and a silicon epitaxial growth process are repeatedly performed with respect to a semiconductor substrate, such as a silicon substrate 10, thereby forming photodiodes 22, 24 and 26 for receiving red light, green light and blue light, respectively.
In detail, an ion implantation layer for the red photodiode 22 is formed in the substrate 10. Then, a first silicon epitaxial layer 12 is formed over the ion implantation layer. After that, the green photodiode 24 is formed in the first silicon epitaxial layer 12 through an ion implantation process. In addition, a plug 42 for a contact of the red photodiode 22 is formed at a predetermined portion of the first silicon epitaxial layer 12.
Then, the silicon epitaxial growth process is again performed to form a second silicon epitaxial layer 14. The blue photodiode 26 is formed in the second silicon epitaxial layer 14. In addition, a plug 46 for a contact of the green photodiode 24 and a plug 44 for a contact of the red photodiode 22 are formed in the second silicon epitaxial layer 14. A plurality of MOS transistors are formed on the second silicon epitaxial layer 14 to transfer photo charges collected in the photodiodes 22, 24 and 26. Each MOS transistor includes a gate 30, a gate insulating layer 32 and a spacer 34.
In such a CMOS image sensor, an area of the red photodiode 22 and the green photodiode 24 is larger than an area of the blue photodiode 26. However, since transfer transistors are aligned at the uppermost position and the blue photodiode 26 is positioned above the red and green photodiodes 24 and 26, the actual light receiving area of the red and green photodiodes 24 and 26 is significantly reduced.
In addition, since the plugs 42, 44 and 46 used for processing signals from the red photodiode 22 or the green photodiode 24 are formed thorough the ion implantation process, signal noise may occur in the plugs 42, 44 and 46 when external light is irradiated thereto.
Moreover, to prevent the red, green and blue photodiodes 22, 24 and 26 from interfering with each other, the red, green and blue photodiodes 22, 24 and 26 must be separated from each other. For this reason, an additional ion implantation layer must be formed in each epitaxial layer to isolate the red, green and blue photodiodes 22, 24 and 26 from each other.